Known in automation- and process control technology are two different measuring principles, which permit determination of fill level of a medium in a container by means of a measuring probe introduced into the container. The firm of Endress+Hausser is active in the field of industrial automation- and process control technology and manufactures industrial measuring devices, often referred to as field devices. Its field devices that utilize container introduced, measuring probes for fill level determination, respectively limit level determination, of a medium in a container are marketed, for instance, under the marks, LEVELFLEX, LIQUICAP and SOLICAP
One of these measuring principles is based on capacitance measurement. In this case, the measuring probe is applied as a capacitive probe, e.g. as an electrode. It is inserted into the container and a capacitance measured for a capacitor formed by the probe and the container wall surrounding the probe. The measured capacitance corresponds to the sum of a basic capacitance of the empty container and the product of a fill substance specific, capacitance increase factor and the fill level of such fill substance.
Another of these measuring principles is based on travel-time measurement. In such case, the fill-level measuring device produces electromagnetic signals, which it sends into the container along a measuring probe serving as a waveguide. A part of these electromagnetic signals is, in turn, reflected back due to a change of the dielectric constant of the medium surrounding the measuring probe at the surface of the fill substance. Such echo signal is received back after a travel time dependent on the fill level. The fill-level measuring device ascertains traveled distance based on the propagation velocity of the measurement signal and the travel-time difference between the transmitting of the measurement signal and the receipt of the echo signal arising from the reflection on the surface of the fill substance. The FMCW method—Frequency Modulated Continuous Waves—, in the case of which the frequency of a continuous measurement signal is continuously changed. Distance is measured using frequency difference between transmitted signal and reflected signal at the moment the reflected signal comes back. The FMCW method is likewise performable as a form of the above travel-time measuring principle with the above described measuring probes as waveguides, surface waveguides or coaxial waveguides.
In the case of time domain reflectometry TDR—Time Domain Reflectometry—, for example, according to the method of the guided microwave, a high-frequency pulse is transmitted along a Sommerfeld waveguide, a Goubau waveguide or a coaxial waveguide. If this electromagnetic signal strikes a surface of the fill substance in the container, then at least a part of the signal is reflected back due to the impedance jump existing at this media boundary. The received signal amplitude as a function of time is the echo signal. Each value of this echo signal corresponds to the amplitude of an echo reflected at a certain distance from the transmitting- and receiving element. The echo signals have marked maxima, which correspond to the portions of the electromagnetic signals, in each case, reflected on the surface of the fill substance. Travel time is ascertained from the time difference between the transmitting of the electromagnetic signal and the receipt of the maxima. Based on the structural dimensions of the measuring arrangement, especially the installed height of the fill-level measuring device in reference to the container, and the propagation velocities of the electromagnetic signals in a medium, e.g. air located above the upper fill substance, there results from the travel time the fill level of the fill substance in the container and therewith the fill level present in the container.
In the following citations, the construction of such measuring probes and the coupling of the measuring signals into these measuring probes are described.
DE 10 2004 060 119 A1 discloses a coupling unit for a time-domain reflectometer, in the case of which the probe element is connected via a threaded connection with the coupling unit of the measuring probe. This construction has the advantage that the probe element can be removed and replaced on-site.
Other combinations of probe elements with coupling units are shown in U.S. Pat. No. 6,178,817 B1, DE 100 45 235 A1 and DE 100 03 941 A1. In the case of this type of measuring probe connection, the probe element is connected with a threaded lug by means of a connecting element outside of the coupling unit, in the process space.
The above described screwed connections of measuring probes are also applicable in the case of a measuring device working according to a capacitive or conductive measuring method. In DE 2003 00 901 U1, a simple measuring probe connection for a capacitive measuring device is described.
Preferably, both for capacitive fill level measurement as well as also for fill level measurement according to the travel time principle, so called coaxial probes are applied as measuring probe units. These comprise an inner conductor as a measuring probe and a tube coaxially surrounding the measuring probe as a shield conductor. Coaxial probes offer the advantage that the measurements executed therewith occur completely independently of the installed situation of the measuring probe in the container. As a result, shape and electrical properties of the container have no influence on the measurement. At the same time, the shielded conductor leads to a maximum signal quality. Influences from external disturbances and loss of power are significantly reduced thereby.
In order to be able to apply such a coaxial probe for a capacitive fill level, measurement and/or a fill level measurement according to the above described travel time principle, it is absolutely necessary that the inner conductor, respectively the measuring probe, be galvanically insulated from the coaxially arranged tube, respectively the shielding conductor and that the shielding conductor lie electrically at a reference potential, preferably ground. For this reason, also in the case of an application in a container filled with an electrically conductive fill substance, there can be no galvanic connection between the inner conductor and the shielding conductor. Such a galvanic connection would lead to a short circuit, which would make both a capacitive fill level measurement as well as also a fill level measurement according to the travel time principle impossible.
According to the state of the art, there are different approaches for achieving releasable, galvanically isolated connections of the measuring probe and the tube arranged coaxially around the measuring probe for the measuring probe unit of a fill-level measuring device. These have, however, the disadvantage that the measuring probe or the coaxially arranged tube can loosen due to vibrations or due to unintended force acting on their connections. If a measuring probe or a tube arranged coaxially around the measuring probe becomes completely detached from its connection to the process connection element, measuring with such measuring probe unit is no longer possible: Furthermore, a dropped measuring probe or a fallen off, coaxial tube of the measuring probe unit in a funnel silo, which has in the lower region, most often, a feed screw or pump, can cause great damage.
Disclosed in DE 10 2006 053 399 A1 is a measuring probe securement by means of a retaining ring or an O-ring, which engages in cavities on the measuring probe and in the measuring probe holder for guarding against accidental release. The engagement of the retaining ring or the O-ring in the cavity in the measuring probe holder can be overcome by application of a predetermined axial tensile force and, thus, the measuring probe can be separated from the process connection element.
A securing against unintentional release of the connection of the tube arranged coaxially around the measuring probe, especially due to arising vibrations of the measuring device or the container, is not shown in the state of the art.